This invention relates to a dielectric barrier discharge lamp.
Of the various low pressure discharge lamps known in the art, the majority are the so-called compact fluorescent lamps. These lamps have a gas fill which also contains small amounts of mercury. Since mercury is a highly poisonous substance, novel types of lamps are being recently developed. One promising candidate to replace mercury-filled fluorescent lamps is the so-called dielectric barrier discharge lamp (shortly DBD lamp). Besides eliminating the mercury, it also offers the advantages of long lifetime and negligible warm-up time and independence of ambient temperature. Concerning these latter two features, a DBD lamp is comparable to an incandescent lamp.
As explained in detail, for example, in U.S. Pat. No. 6,060,828, the operating principle of DBD lamps is based on a gas discharge in a noble gas (typically Xenon). The discharge is maintained through a pair of electrodes, of which at least one is covered with a dielectric layer. An AC voltage of a few IV with a frequency in the kHz range is applied to the electrode pair. Often, multiple electrodes with a first polarity are associated to a single electrode having the opposite polarity. During the discharge, exciters (excited molecules) are generated in the gas, and electromagnetic radiation is emitted when the meat-stable exciters dissolve. The electromagnetic radiation of the exciters is converted into visible light by suitable phosphors, in a physical process similar to that occurring in mercury-filled fluorescent lamps. This type of discharge is also referred to as di electrically impeded discharge.
As mentioned above, DBD lamps must have at least one electrode set which is separated from the discharge gas by a dielectric. It is known to employ the wall of the discharge vessel itself as the dielectric. Various discharge vessel-electrode configurations have been proposed to satisfy this requirement. U.S. Pat. No. 5,994,849 discloses a planar configuration, where the wall of the discharge vessel acts as a dielectric. The electrodes with opposite polarities are positioned alternating to each other. The arrangement has the advantage that the discharge volume is not covered by electrodes from at least one side, but a large proportion of the energy used to establish the electric field between the electrodes is dissipated outside the discharge vessel. On the other hand, a planar lamp configuration can not be used in the majority of existing lamp sockets and lamp housings, which were designed for traditional incandescent bulbs.
In order to increase the efficiency, it has been proposed to put the electrodes within the discharge vessel, to lower the dissipation losses occurring outside the discharge vessel. U.S. Pat. Nos. 6,034,470 and 6,304,028 disclose two different DBD lamp configurations, where both set of electrodes are located within the discharge vessel, which confines the discharge gas atmosphere. The electrodes are covered with a thin layer of dielectric. However, none of these lamp configurations are suitable for a low-cost mass production, because the thin dielectric layers need an additional process step, and they are prone to premature aging, which quickly destroys their insulating properties.
U.S. Pat. No. 5,763,999 and U.S. Patent application Publication No. US 2002/0067130 A1 disclose DBD light source configurations with an elongated and annular discharge vessel. The annular discharge vessel is essentially a double-walled cylindrical vessel, where the discharge volume is confined between two concentric cylinders having different diameters. A first set of electrodes is surrounded by the annular discharge vessel, so that the first set of electrodes is within the smaller cylinder, while a second set of electrodes is located on the external surface of the discharge vessel, i.e. on the outside of the larger cylinder.
This known arrangement has the advantage that none of the electrode sets need any particular insulation from the discharge volume, because the walls of the discharge vessel provide stable and reliable insulation. However, the external electrodes are visually unattractive, block a portion of the light, and also need to be insulated from external contact, due to the high voltage fed to them.
U.S. Pat. No. 6,246,171 B1 also discloses discharge vessel-electrode configurations where both the first and second sets of electrodes are located on the same side of a discharge vessel wall, similar to that proposed in U.S. Pat. No. 5,994,849. However, this configuration has the inherent disadvantage that the intensity of the electric field within the discharge volume is relatively small, and this negatively affects the efficiency of the lamp. On the contrary, the stray electric field (i.e. the field which is outside of the discharge volume, and hence useless for the purposes of the discharge) is relatively large. Therefore, U.S. Pat. No. 6,246,171 B1 also proposes to place the electrodes on two opposing surfaces of the discharge vessel, enclosing the discharge volume between the opposing surfaces, similarly to the solutions described above, albeit not for an annular discharge vessel but for a flat radiator. In this manner, a larger portion of the electric field will penetrate the discharge volume, and will contribute more effectively to the discharge. However, this arrangement again has the disadvantage that the electrodes will be visible from that side onto which they were applied.
Therefore, there is a need for a DBD lamp configuration with an improved discharge vessel-electrode configuration, which does not interfere with the aesthetic appearance of the lamp. There is also need for an improved discharge vessel-electrode configuration which ensures that the electric field within the discharge volume is homogenous and strong, and thereby effectively contributes to the barrier discharge. It is sought to provide a DBD lamp, which, beside having an improved electrode-discharge vessel arrangement, is relatively simple to manufacture, and which does not require expensive thin-film dielectric layer insulations of the electrodes and the associated complicated manufacturing facilities. Further, it is sought to provide a discharge vessel which readily supports electrode sets which are easy to apply directly onto the discharge vessel walls, but which will still have a reduced stray electric field.